The invention relates to a circuit device for operating a discharge lamp by means of a high-frequency current, which circuit device comprises input terminals for connecting it to a low-frequency supply-voltage source, rectifier means which are coupled to the input terminals and which are used to rectify the low-frequency supply voltage, a first circuit coupled to a first output terminal N3 of the rectifier means and to a second output terminal N5 of the rectifier means, which first circuit comprises a series arrangement of first unidirectional means, second unidirectional means and first capacitive means, which circuit device further comprises inverter means which are connected in parallel with the first capacitive means and which serve to generate the high-frequency current, a load circuit comprising a series arrangement of inductive means, second capacitive means and means for applying a voltage to the discharge lamp, which load circuit connects a terminal N1 of the inverter means to a terminal N2 between the first unidirectional means and the second unidirectional means, which circuit device further comprises a second circuit connecting a terminal N2 to terminal N5 and comprising third capacitive means, and a third circuit connecting the first output terminal N3 of the rectifier means to a terminal N4 between the second unidirectional means and the first capacitive means and comprising a series arrangement of third unidirectional means and fourth unidirectional means, wherein neither the first circuit nor the third circuit include inductive means, and a fourth circuit connecting the third circuit to a terminal N6, which forms part of the load circuit.
Such a circuit device is disclosed in WO 97/19578.
The known circuit device is dimensioned such that the power feedback circuit is optimized for a certain power level with respect to total harmonic distortion and power balance. This means of course that at a power level which deviates from the predetermined power level, the total harmonic distortion and the power balance are sub-optimal; this is certainly the case, for example, at a difference of 40% in the case of different power levels of 100% and 60%.
It is an object of the invention to improve the known circuit device in such a manner that optimization with respect to total harmonic distortion and power balance is possible for more than one power level.
To achieve this, the invention provides a circuit device of the type mentioned in the opening paragraph, which is characterized in that the third circuit further comprises fifth unidirectional means interposed between the fourth unidirectional means and terminal N4, terminal N4 is coupled to a terminal N8 between the fourth and the fifth unidirectional means via a series arrangement of sixth unidirectional means and switching means, said sixth unidirectional means being oppositely directed relative to the third, fourth and fifth unidirectional means which are equally directed, and terminal N8 is connected to terminal N6 by means of a fifth circuit.
By virtue of said measures, the circuit device can be selectively adapted to different power levels while maintaining optimum total harmonic distortion and power balance.
It has been found that smooth operation of the circuit device can be achieved if the fifth circuit comprises fifth capacitive means.
In addition, capacitive voltage division enables optimum adaptation to discharge lamps to be used.
Furthermore, the invention enables optimization regarding total harmonic distortion and power balance to be achieved for more than two power levels by embodying a sub-circuit device comprising the fourth, fifth and sixth unidirectional means, the switching means and the fifth circuit so as to form a multiple sub-circuit device. The series connections of fourth and fifth unidirectional means of the multiple sub-circuit device are series-connected between terminals N7 and N4 of the circuit device and have a respective terminal N8i. The series connections of sixth unidirectional means and switching means of the multiple sub-circuit device are connected between the respective terminal N8i and terminal N4 of the circuit device. The fifth circuits of the multiple sub-circuit device are connected between the respective terminal N8i and terminal N6 of the circuit device. The fifth and the fourth unidirectional means may then be formed from common unidirectional means.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.